This invention relates to a method of developing presensitized offset printing plates and a developing solution used in that method.
Aluminum plates are commonly used as supports for presensitized offset printing plates. Since alkaline solutions having high pHs are used to develop presensitized offset printing plates, the surface of the aluminum plate that becomes exposed in areas from which the light-sensitive layer has been removed tends to be corroded by alkaline developing solutions to impair the hydrophilic nature of the non-image areas, thereby increasing the chance of fouling of the print. Another problem with the corroded aluminum plate is sludge formation, which can either foul the automatic processor or cause sludge deposition on the printing plate being developed. These phenomena are particularly noticeable when using a developing solution that contains a silicate in a small amount (.ltoreq.1.0 wt % as SiO.sub.2) and that has a pH of at least 12.5.
As a method for preventing the corrosion of aluminum supports by alkaline developing solutions, Unexamined Published Japanese Patent Application No. 59444/1983 teaches the use of an alkaline developing solution that contains both sodium metasilicate and an inorganic lithium compound. Unexamined Published Japanese Patent Application No. 25100/1980 proposes that an ionizable compound of an element of the group IIa, IIIa or IIIb of the periodic table be added to the developing solution. However, neither of these methods are completely satisfactory for the purpose of preventing the corrosion of aluminum supports.